This invention relates to devices and methods for determining the inner pressure of an eye. More specifically, this invention relates to a device and a method for determining the inner pressure of a human eye and a measuring sensor for capturing measurement values for determining the inner pressure of a human eye.
The inner pressure of the eye or so-called intraocular pressure (IOP) is maintained through a regulated flow of water in the eye chamber. In ophthalmology, the measurement of the intraocular pressure is of great importance for medical diagnosis. Too low an intraocular pressure leads to destabilization of the eye. Too high an intraocular pressure can lead to glaucoma, a very diverse group of diseases of the eye in which the IOP is too high for a normal, lasting functioning of the optic disk. The intraocular pressure is considered at the present time to be the most important and most easily measured parameter in the treatment and early diagnosis of glaucoma. Statistical records show, for example, an average intraocular pressure of 15.9 mm Hg (1 mm Hg=1.33 mbar) for men and 16.6 mm Hg for women. The intraocular pressure is subject both to daily fluctuations of 3 to 6 mm Hg and to fluctuations through blood pressure (pulse) of about 1.5 mm Hg. Although the intraocular pressure differs from individual to individual, 21 mm Hg is often set as the limit value at which at least further medical clarification of glaucoma risk is considered necessary. Furthermore a daily fluctuation of the IOP by 10 mm Hg is considered pathological. In the latest approaches, even a change in the brief fluctuations of the IOP caused by the pulse are examined for glaucoma.
The intraocular pressure is measured by so-called tonometers. In the known tonometry methods, the pressure is determined through measurement of force with known area or through measurement of area with known force. There are also tonometry methods in which an external pressure is applied to the eye and the applied pressure is measured. In addition, there are non-contacting tonometers which generate a flattening of the cornea by means of an air blast, and the IOP is thereby able to be measured. The most widespread tonometer at the present time and the one considered to be the most precise, the Goldmann applanation tonometer, is based on a force measurement with constant contact surface area (with flat contacting, this is referred to as the applanation surface). With Goldmann applanation tonometers, the size of the applanation surface is selected such that the forces caused by the cornea stiffness and the adhesion forces, caused by the meniscus of the tear fluid between the applanation surface and the eye, offset one another, which is the case for circular applanation surfaces having a diameter of 3 to 4 mm. With Goldmann applanation tonometers, the predefined size of the applanation surface is set by increasing the force applied to the applanation surface until two semicircles touch when looking through the transparent applanation element provided with a double prism. The intraocular pressure IOP then results from the ratio F/A of the measured force F applied to the applanation surface and the set predefined applanation surface area A. Goldmann applanation tonometers are used with a slit lamp, and are typically designed as standing apparatus so that the patient has to be seated, and self tonometry, performed by the patient himself, is not possible. Hand-held embodiments of the Goldmann applanation tonometer are achieved in a way still more costly with respect to the technology of the device. The technical investment with respect to apparatus is very high with the Goldmann applanation tonometer, above all if the measurement process is automated, as is necessary for self-tonometry by the patient, since the applanation surface area has to be determined and set automatically.
Described in the published patent application WO 00/71982 is a newer device for determination of the intraocular pressure which is designed as a simple-to-insert contact lens and which therefore seems that it could be suitable for self-tonometry by the patient. The contact lens according to WO 00/71982 has a concave recess whose surface contour is adapted to the curvature of the human eye. A bore, sealed by a membrane, is provided at the vertex of the recess. The bore is part of a fluid-filled chamber system which transmits to a pressure measuring unit, located in the chamber system, a pressure exerted on the membrane, the fluid serving as a pressure transmission medium. Owing to the shape of the device according to WO 00/71982, the intraocular pressure can be greatly influenced, however, by a placement on the eye not according to directions (gently and with no application of force) and by periodic lifting of the device from the eye so that measurement values result indicating an incorrect IOP. This definitely makes self-tonometry by the patient more difficult.
To obtain a correct measurement result, the known tonometers must be applied very precisely, in particular perpendicular to the cornea, and carefully. Moreover, since some of the known tonometers only indicate the momentary value of the IOP, several measurements need to be carried out to determine the average IOP. Furthermore the known tonometers, when compared to one another, give differing measurement results. The thickness, the stiffness and the shape of the cornea influence the measurement results for the IOP in many methods. To take into consideration the corneal thickness, attempts are being made, among other things, to combine the IOP measurement with a measurement of the corneal thickness.
It is an object of this invention to propose a device and a method for determining the inner pressure of the eye, which device does not have at least certain of the above-described drawbacks of the state of the art and which makes possible, in particular, determination of the intraocular pressure with spatial resolution.
In particular, these objects are achieved in that the device for determining the intraocular pressure comprises a plurality of pressure-sensing elements that are placed directly or indirectly on the eye, and it comprises processing means for determining the intraocular pressure from the sensor pressure values measured by the individual pressure-sensing elements. The pressure-sensing elements are preferably disposed in a pressure sensor array which is placed directly or indirectly on the eye, the pressure sensor array comprising at least one line, but preferably a plurality of lines, of pressure-sensing elements. The advantage of using a plurality of pressure-sensing elements, in particular a pressure sensor array, for determining the intraocular pressure is above all that the intraocular pressure can be determined with spatial resolution. Irregularities in the local pressure distribution can thereby be recognized, and, for example, a poor application, i.e. an incorrect placement of the pressure sensor array on the eye, can be recognized as an invalid measurement. In particular, too forceful or too weak contacting (in the case of planar contacting, this is called applanation) of the eye during placement of the pressure sensor array can be recognized and can be indicated to the user. Since neither a predefined size of the contact surface (applanation surface in the case of planar contacting) nor a predefined pressing force has to be set, patients also need to be positioned less still and aligned. Moreover, in the case of a flat arrangement of the pressure-sensing elements, an especially simple application of the device is made possible since only a flat surface (pressure sensor array) has to be brought into contact with a spherical section (cornea), and special attention does not have to be paid to a centering of the contact surface and to a perpendicular application of the device, as with some state-of-the-art methods. A further advantage is that use of a pressure sensor array makes possible determination of the intraocular pressure without moving mechanical parts.
The processing means are preferably designed such that they determine the inner pressure of the eye from the sum of the measured, and e.g. weighted, sensor pressure values and the number of pressure-sensing elements contributing to the sum. Since the intraocular pressure is calculated through a division of the sum of the measured sensor pressure values by the number of contributing (active) pressure-sensing elements, the pressure sensor array does not have to rest on the eye with all its pressure-sensing elements. From this results the advantage that different contact conditions are suitable for determining the intraocular pressure, and, for example, oval contact surfaces, such as arise with astigmatism, do not falsify the determination of the intraocular pressure.
In an embodiment variant, the device comprises filter means to exclude, according to predefined criteria, certain measured sensor pressure values from the summation. Individual sensor pressure values which unfavorably influence the calculation of the intraocular pressure can thereby be excluded from the determination of the inner pressure of the eye. For example, sensor pressure values that are measured by pressure-sensing elements on the outer edge of the contact area and that are influenced by the stiffness of the cornea and/or by the adhesion forces of the lacrimal meniscus between the eye and the respective pressure sensor element, can be excluded from the determination of the intraocular pressure. Sensor pressure values can also be excluded that would otherwise falsify the determination of the intraocular pressure owing to variable corneal thickness of the respective eye, as a result of local corneal thickening or depression, for instance. It is also possible to detect irregularities caused by the corneal stiffness and to use them to compensate for the influences of the corneal stiffness. The exclusion of certain measured sensor pressure values can be also advantageous when the pressure sensor array used has a coarse spatial resolution, i.e. when the pressure sensor array is made up of few pressure-sensing elements which each measure a relatively large contact surface, so that individual pressure sensor elements on the outer edge of the contact area rest only partially on the eye.
In an embodiment variant, the device comprises memory means for storing of the measured sensor pressure values and for storing a multiplicity of certain values of the inner pressure of the eye. The capturing (measuring) of the sensor pressure values and the evaluation of the captured sensor pressure values can thereby be carried out in different steps, shifted with respect to time, which can also facilitate self-tonometry by the patient, for example. The storing of the measured sensor pressure values also makes calculation easier of the temporal averages for the measured sensor pressure values. Moreover the amplitude of the pressure fluctuation of the IOP caused by the pulse of the blood pressure can also be recorded. The storing of a multiplicity of particular inner pressure values for the eye, each preferably together with associated time indications, makes possible moreover the comparison of intraocular pressure values taken at different times.
In an embodiment variant, the device comprises representation means for graphic illustration and in particular depiction with spatial resolution of the measured and/or stored sensor pressure values. Pressure distribution profiles along a line through the contact surface and/or two-dimensional pressure distribution profiles over the entire contact surface, e.g. pressure distribution matrices, can thereby be shown and analyzed. By means of the sensor pressure values stored through graphic representation, sequences of pressure distribution profiles and pressure distribution matrices can also be shown and analyzed.
In an embodiment variant, the device comprises communications means for remote transmission of the determined inner pressure values for the eye. It is thereby made possible, for example, for the inner pressure values for the eye determined by a patient through self-tonometry to be transmitted to a responsible physician for recording and evaluation.
The pressure sensor array is preferably designed as a micro-electromechanical system (MEMS). A high degree of integration and thus a high degree of miniaturization are thereby made possible so that compact, portable devices can be achieved with a high spatial resolution.
In an embodiment variant, the pressure sensor array is incorporated into a measuring sensor that is connected to an evaluation unit via an interface having contacts or a contactless interface. Especially compact measuring sensors can thereby be achieved which can be combined via corresponding interfaces and programmed software modules with conventional processing and display units, for instance with palmtop, laptop or personal computers or with mobile radio telephones.